;+ ; NAME: ; CHIPSI_TO_THETAPHI ; ; PURPOSE: ; Converts coordinates from instrument chi-psi to ECR theta-phi ; ; CATEGORY: ; Utility ; ; CALLING SEQUENCE: ; chipsi_to_thetaphi, satlon, chi, psi, theta, phi ; ; INPUTS: ; satlon Satellite longitude, any type ; chi Instrument look angle from nadir toward east, any type ; psi Instrument look angle from nadir toward north, any type ; ; OUTPUTS: ; theta Zenith angle of observation in ECR coordinate system, double precision ; phi Azimuthal angle of observation in ECR coordinate system, double precision ; ; KEYWORDS: ; None ; ; COMMON BLOCKS: ; None. ; ; PROCEDURE: ; Simple transform from instrument look angles to Earth-Centered Rotational (ECR)i ; coordinate system assuming a geostationary satellite (or any equatorial location). ; The chi-psi coordinate system is defined as look angles from nadir. ; Note this essentially assumes that nadir is in the -z direction for a right-hand system. ; S/C coordinates, however, generally and perversely assume that nadir is +z. ; Therefore, in this type of S/C coordinates, chi is +x but psi is -y. ; Arrays of coordiantes of any dimension are OK. ; ; ROUTINES USED: ; None. ; ; MODIFICATION HISTORY: ; Stan Solomon, 3/15 ; ;- pro chipsi_to_thetaphi, satlon, chi, psi, theta, phi theta = !dpi/2. - psi phi = !dpi + satlon - chi return end